1. Technical Field
The present disclosure relates to a transmission apparatus and a transmission method.
2. Description of the Related Art
In wireless communication, a low cost and low power consumption transmission apparatus is needed. To reduce power consumption, replacement of power amplifiers of high power consumption is effective. One of low power consumption power amplifiers is a class-D amplifier.
Class-D amplifiers have a structure that alternately turns on the switch between the power source and the output and the switch between the ground and the output and performs an output. Since there is ideally no current consumption other than current flowing through the load, class-D amplifiers have high power efficiency.
However, conventional class-D amplifier have the following technical problems concerning the control method for output power and its linearity.
Since an output of a class-D amplifier is ideally connected to the power source or ground through a switch, the amplitude of the output voltage swings between the power source and the ground. Accordingly, the amplitude of the output voltage can be controlled by changing the power voltage, but a low noise and high response low drop-out (LDO) regulator is necessary to change the power voltage. Since a high speed LDO changes the voltage through a resistance loss of a transistor, a power loss occurs. In addition, when the power voltage is lowered, the ON resistance of the above switch increases and the linearity of the amplitude of the output voltage relative to the power voltage becomes worse, thereby degrading output signal distortion properties during inputting of a modulation signal with amplitude variations.
A solution to these problems is a switched capacitor power amplifier (referred to below as an SCPA) (see “A Switched-Capacitor RF Power Amplifier”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 46, NO. 12, DECEMBER 2011). An SCPA solves the above problems while keeping the high efficiency of a class-D amplifier.
The equivalent circuit of an SCPA has a structure in which a plurality of class-D amplifiers each having a series capacitor connected to the AND output are connected in parallel. Of the class-D amplifiers connected in parallel, a number of class-D amplifiers proportional to an output power control signal reach the operating state and the AND outputs of the other class-D amplifiers are fixed to the ground electric potential. In this structure, output signals from the class-D amplifiers in the operating state are transferred to the output side through the series capacitors and are also transferred to the ground through the series capacitors of the class-D amplifiers in the stop state. That is, the output signals from the class-D amplifiers in the operating state are transferred to the output side and the output power is determined by the capacitance ratio.
In implementation of an SCPA on a semiconductor, since high relative precision is expected even though there are variations in manufacturing, it is possible to obtain high linearity and wide output power variable range without performing correction. Since the series capacitor of the AND output is always connected to the power source or the grounding point, which is the ground, regardless of whether the class-D amplifier is in the operating state or the stop state, the output impedance is constant regardless of the output power control signal. Consequently, the LC resonance frequency with the inductor is constant regardless of the output power control signal and fundamental frequency components can be extracted stably from the output signal.
As shown in FIG. 1, the transmission apparatus disclosed in “All-Digital RF I/Q Modulator”, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 60, NO. 11, NOVEMBER 2012 achieves the function of a power amplifier in addition to the function of a quadrature modulator by using two digital power amplifiers as a mixer for an in-phase component (I) and a quadrature component (Q). At the output of the power amplifier, being converted into an analog signal, the signal is a digital signal, so the analog mixer, DA converter, and post filter for an in-phase component (BB_I) and a quadrature component (BB_Q), which are necessary for a conventional analog quadrature modulator, become unnecessary. Since most of signal processing is digital signal processing, cost reduction can be achieved using recent fine semiconductor processes. In addition, since 25%-duty signals are used as carrier signals, either an in-phase component (BB_I) or a quadrature component (BB_Q) is connected to the output of the power amplifier. As a result, when the in-phase component (BB_I) is synthesized with the quadrature component (BB_Q), the individual power amplifier outputs can be connected directly without using a balun (a balun is used for difference/one-phase conversion in the above document).